The following are conventional methods for manufacturing pellets for making electromagnetic wave shielding material with conductive fibers dispersed in a matrix resin:
(1) A method for manufacturing pellets through the kneading of a matrix resin and conductive fibers, cut about 5 mm long by, for example, an extruder and the dispersing of the conductive fibers in the matrix resin.
(2) A method for manufacturing pellets through the coating of a strand of conductive fibers directly with a matrix resin by an extruder and the cutting of the strand to a desired length.
(3) A method for manufacturing pellets by dipping a strand made up of conductive fibers into a hot-melt resin, to coat it with the resin and by cutting the coated strand into predetermined lengths.
(4) A method for manufacturing pellets by dipping conductive fibers into a solution of a matrix resin in a solvent to coat them with the matrix resin and by cutting them to a desired length.
In the method (1), during the kneading of the conductive fibers and resin by the kneader, the conductive fibers are cut shorter than their initially cut length due to the direct cutting of the conductive fibers by a screw and due to the shearing of the resin. Furthermore, where an electromagnetic wave shielding material is formed through the molding of the kneaded material, such as an extrusion molding or injection molding, the resultant conductive fibers become much shorter. Because of the dispersion of the thus-shortened conductive fibers (0.5 mm or below) in the resin it is not yet possible to obtain a practical shield effect due to an increase in multiple contacts among the conductive fibers and to the consequent increase of electrical resistance of the electromagnetic wave shielding material. In order to improve the shield effect, more conductive fibers need to be added to the resin, resulting in high costs. According to the method (2), it is possible to prevent the undesirable cutting of the conductive fibers. However, it is difficult to disperse the conductive fibers in the resin during the molding process. This is because the conductive fibers are not uniformly mixed in the resin due to a lessening of the wettability of the conductive fibers and matrix resin. It is not possible to obtain a shield effect because of the increased electrical resistance involved. Method (3) involves poor workability and is never economical. Method (4) cannot obtain the desired surface configuration and is high in costs because of the use of the solvent.